Motor voltage and frequency controls based on estimates of the motor speed and load are generally known in the motor control art. For example, it is known that the motor voltage can be boosted (increased) at relatively low speed to improve the motor output torque. As an efficiency measure, however, the voltage may be scaled back when the motor load is relatively light. Moreover, the frequency of the applied voltage may be increased with load to compensate for load-related slippage.
It is academic that the load of the motor, as a percentage of the full or rated load torque, is given by the expression: EQU % LOAD=[(V.sub.bus * I.sub.bus)-LOSSES)]/(IF/60 * HP)
where the product (V.sub.bus * I.sub.bus) represents the input power supplied to the motor drive electronics, LOSSES represents the power losses of the motor and drive electronics, IF/60 is the ratio of the inverter frequency IF to the rated motor speed of 60 Hz, and HP is the rated power of the motor in Watts. Unfortunately, however, the various terms are interrelated. For example, the frequency and voltage controls are based on load, and yet directly affect the load calculation. Moreover, the power losses used in the load computation vary with load as well.
Due to the above, prior motor control systems have achieved the voltage and frequency control based on estimations of the motor load. Unfortunately, the estimations are often relatively crude, yielding good load estimation under some conditions, and poor load estimation under other conditions.